jQuery(document).ready(function($) {// dollar passed in so you can use it as a short reference in script
    //==== Hide and disable your chosen fields/containers when doc ready====//
    //Disable Field
    $('#my_field').prop('disabled', true);
    //Hide Container
    $("#my_field_container").hide();

    //==== Handle the click function ====//
    $("#radio_field").click(function() {
      //initialise var with the current value of radio button 
      var radioVal= $("#radio_field input:radio:checked").val();

      if(radioVal == the_value_you_want_to_show_on){
         //Show the container & enable the field]
         $("#my_field").prop('disabled', false);
         $("#my_field_container").show();
      }else{
         //Hide the container & disable the field
         $("#my_field").prop('disabled', true);
         $("#my_field_container").hide();    
      }//end hide/disable
    });//end .click()

});//end document ready

<?php
/**                               ______________________________________________
*                          o O   |                                              |
*                 (((((  o      <    Generated with Cook Self Service  V2.0     |
*                ( o o )         |______________________________________________|
* --------oOOO-----(_)-----OOOo---------------------------------- www.j-cook.pro --- +
* @version		1.0
* @package		Franchisers
* @subpackage	Uizendkrachten
* @copyright	Copyright (C) 2012. All rights reserved.
* @author		Gosling Cools | COBIZ webdevelopment - http://www.cobiz.nl - info@cobiz.nl
* @license		GNU General Public License version 2 or later
*
* /!\  Joomla! is free software.
* This version may have been modified pursuant to the GNU General Public License,
* and as distributed it includes or is derivative of works licensed under the
* GNU General Public License or other free or open source software licenses.
*
*             .oooO  Oooo.     See COPYRIGHT.php for copyright notices and details.
*             (   )  (   )
* -------------\ (----) /----------------------------------------------------------- +
*               \_)  (_/
*/

// no direct access
defined('_JEXEC') or die('Restricted access');


cimport('classes.upload');
$fieldSets = $this->form->getFieldsets();
?>
<?php $fieldSet = $this->form->getFieldset('uizendkracht.form');?>
<fieldset class="fieldsform">
	<dl>
<?php
// Uitzendkracht Naam
$field = $fieldSet['jform_u_naam'];
?>
	<dt><?php echo $field->label; ?></dt>
    <dd><?php echo $field->input; ?></dd>



<?php
// Email
$field = $fieldSet['jform_u_email'];
?>
<span class="email_container">
	<dt><?php echo $field->label; ?></dt>
    <dd><?php echo $field->input; ?></dd>
</span>


<?php
// Klanten ID
$field = $fieldSet['jform_klanten_id'];
$field->jdomOptions = array(
			'list' => $this->lists['fk']['klanten_id']
		);
?>
	<dt><?php echo $field->label; ?></dt>
    <dd><?php echo $field->input; ?></dd>



<?php
// My Enumeration
$field = $fieldSet['jform_my_enumeration'];
$field->jdomOptions = array(
			'list' => $this->lists['select']['my_enumeration']->list
		);
?>
	<dt><?php echo $field->label; ?></dt>
    <dd><?php echo $field->input; ?></dd>



<?php
// PDF
$field = $fieldSet['jform_pdf'];
$field->jdomOptions = array(
			'actions' => array('remove', 'thumbs', 'delete', 'trash')
		);
?>
	<dt><?php echo $field->label; ?></dt>
    <dd><?php echo $field->input; ?></dd>

	</dl>
</fieldset>

<script type="text/javascript">
jQuery(document).ready(function($) {// dollar passed in so you can use it as a short reference in script
    //==== Hide and disable your chosen fields/containers when doc ready====//
    //Disable Field
    $('#jform_u_email').prop('disabled', true);
    //Hide Container
    $(".email_container").hide();

    //==== Handle the click function ====//
    $("#__jform_my_enumeration").click(function() {
      //initialise var with the current value of radio button 
      var radioVal= $("#__jform_my_enumeration input:radio:checked").val();

      if(radioVal == 'deheer'){
         //Show the container & enable the field]
         $("#jform_u_email").prop('disabled', false);
         $(".email_container").show();
      }else{
         //Hide the container & disable the field
         $("#jform_u_email").prop('disabled', true);
         $(".email_container").hide();    
      }//end hide/disable
    });//end .click()

});//end document ready
</script>

function toggleShowHideDisableEnable(myDiv, myField, value){
  //Do stuff... Pretty much the same as before except that
  // you replace the id strings in the script with the parameters passed in
}

<field name="u_email"
				label="FRANCHISERS_FIELD_EMAIL"
				alias="u_email"
				required="true"
				filter="STRING"
				class="validate[required,custom[email]]"
				validate="email"
				type="cktext"/>

jQuery(document).ready(function($) {// dollar passed in so you can use it as a short reference in script
    //==== Hide and disable your chosen fields/containers when doc ready====//
    //Disable Required
	$("#jform_u_email").removeClass('inputbox validate[required,custom[email]]');
    //Hide Container
	$(".email_container").hide();

    //==== Handle the click function ====//
    $("#__jform_my_enumeration").click(function() {
      //initialise var with the current value of radio button 
      var radioVal= $("#__jform_my_enumeration input:radio:checked").val();

      if(radioVal == 'deheer'){
         //Show the container & enable the Required
		 $("#jform_u_email").addClass('inputbox validate[required,custom[email]]');		 
         $(".email_container").show();
      }else{
         //Hide the container & disable the Required
		 $("#jform_u_email").removeClass('inputbox validate[required,custom[email]]');
         $(".email_container").hide();
		 
      }//end hide/disable
    });//end .click()

});//end document ready

$("#jform_u_email").removeClass('required');

$("#jform_u_email").addClass('required');